Various environmental and industrial chemicals can disturb male reproductive function. The objective of these studies is to enhance our understanding of that toxic potential, and to elucidate the mechanism of action in chemicals found to be toxic. Chemicals which are active as chemosterilants in males, such as glycol ethers, dimethyl methyl phosphonate, dibromochloropropane and the phthalate esters, are used in various test systems. In addition to mechanistic studies, chemicals of unknown activity, such as the phenoxy herbicides and TCDD, have also been studied. Endpoints of toxicity include the assessment of testicular morphology, spermatogenesis, sperm morphology, and hormone levels. Studies continue on the morphological response of the testis to chemical exposure. Androgen Binding Protein (ABP) assays will also be performed to assess Sertoli cell function. Studies are beginning which will evaluate a marker for germ cell response to testicular toxicants. Other studies are beginning to examine any changes of Sertoli cell function in vitro after exposure to toxicants in vivo and in vitro. Cell separation studies will examine biochemical defense mechanisms in isolated germ cell populations after exposure to various toxicants. These studies are expected to yield valuable data on chemical toxicity in males, as well as improve the sensitivity and accuracy of future testing systems.